This invention relates in general to cushions and more particularly to wheelchair cushions that are structured to redistribute load under tissues that support the bony prominences of the pelvis, such as the ischial tuberosities and the trochanters.
Wheelchair cushions that redistribute load are beneficial to users who are at risk of developing pressure ulcers due to prolonged sitting. In addition to redistributing pressure, wheelchair cushions should provide a stable base for sitting. This is particularly true for users who have compromised trunk stability due to neuromuscular deficits.
Wheelchair cushions use foam and other compressible materials to distribute pressure by compressing and therefore increasing the area over which the load is distributed. Many cushions have been developed that have varying stiffness of compressible materials, specifically foam. As an example, a cushion may be designed with softer material under the ischia and firmer material under the trochanters. The softer material compresses to allow the ischia to immerse, thereby allowing the trochanters to bear load. The firmer material does not compress as easily, so that when the user leans laterally, the material will resist compression and provide stability.
Another design of wheelchair cushions uses fluid, contained in a bladder, that is displaced under load. Fluid cushions are designed to allow the fluid to flow in response to loading. The natural movement of fluid results in flow from areas of high load to areas of lower load. Cushions use design elements to manage and control the flow in order to maintain a stable sitting base. For example, when a user leans to one side, the fluid would rapidly flow away from the increased load and create instability. The design of some fluid cushions is such that this flow is restricted. For example, a cushion that uses air as the fluid may restrict the flow of air between cells to increase the time required to flow from cell to cell. As a result, when a user leans to the left or right, the air does not immediately move away from the increased load, which would cause instability. Other cushions use specialty fluids that will flow gradually in response to pressure, but retain their shape and position in the absence of pressure. Again, by restricting flow over time, the cushions can improve stability.
In general, the market sees well-designed fluid cushions as superior to well-designed foam cushions. While foam cushions rely on deflection and compression to relieve pressure on the ischia, fluid cushions allow for fluid displacement and hydrostatic loading. This is advantageous for multiple reasons. First, in general, well-designed fluid cushions retain supportive properties over time, compared to foam cushions, which gradually break down. Second, there is less shear stress on the skin tissue while displacing fluid than while deflecting foam. Last, as mentioned previously, viscous, non-Newtonian fluids maintain their position in the absence of pressure and create more stability for the user.